Traditional total knee arthroplasty instruments utilize intramedullary instruments to determine proper distal femur saw cut alignment, and extramedullary instruments to align the saw cut for the proximal tibia. Therefore it is acceptable to prepare the distal femur separate from the proximal tibia. There exists no conjoined effort to cut the distal femur and the proximal tibia as the single lower extremity body part which constitutes the knee joint.
This contemporary instrumentation process violates the principles established by Insall in the 1970s. Popular total knee arthroplasty instruments teaches this inexact intramedullary instrument process because it is simpler to teach, understand and utilize by most surgeons.
Dr. Insall recognized the need for external rotation (ER) of the femoral component when performing a total knee arthroplasty (TKA). In 1990 Dr. Insall attributed the need for approximately 3 degrees of ER to an “abundance of soft tissue in the posteromedial corner of the knee.”
Indeed in the absence of this prescribed ER of the femoral component 1) patellar tracking will be unbalanced, related to the trochlear groove and 2) the medial compartment will be compressed significantly greater than the lateral compartment with the knee flexed beyond 40 degrees and 3) the patella would track laterally.
The reason for alteration of the normal morphology of the distal femur when performing a TKA is not well understood.
The reason for the need to externally rotate the femoral condyle approximately 3 degrees relative to the normal morphology of the femoral condyles is the clue to surgical alteration of normal morphology of the proximal tibia.
Normal Anatomy of the Proximal Tibia
As is well known, in a lateral xray of a normal proximal tibia, the plane of the medial tibial plateau exists approximately 3 mm more distal than the lateral tibial plateau.
Evident in a CAT scan of a normal knee is the elevation difference between the planes of the two tibial plateaus.
If a saw cut is made at the proximal tibia, at a right angle to the vertical axis of the tibia, the medial tibial compartment will be elevated relative to the lateral tibial plateau. This relative elevation will, in turn, elevate the medial femoral condyle, necessitating removal of an equal amount of posterior medial femoral condyle (equal to the relative elevation of the medial tibial plateau) in order to maintain proper tracking of the patellar throughout flexion and extension of the knee. It is the external rotation of approximately 3 degrees (3 mm) that accomplishes about 3 mm more removal of the condyle on the medial side than the lateral side.
The most common adjustment position for “external rotation guides” is 3 degrees. This position will remove about 3 mm more off the medial femoral condyle than the lateral femoral condyle. The reality is, and therefore the error is, that condylar and plateau articular cartilage wear, and differences in plateau height between the medial and lateral plateaus, will require external rotation adjustments between 1 degree and 6 degrees in order to balance compression forces in the medial and lateral compartments for both flexion and extension.
It is only after equal compartment compression is accomplished through proper external rotation that proper ligament releases can be accomplished.
Method for Getting External Rotation Right
Equal compression of the medial and lateral compartments can only be obtained by causing the posterior femoral condylar cut to be parallel to the proximal tibial cut.
To accomplish this:
Pin the tibial cut guide in place with the tibial alignment rod centering distally over the middle of the plafond. The plafond is the ceiling of the ankle joint, that is, the articular surface of the distal end of the tibia.
After resecting the distal femur, place the 4-in-1 femoral cut guide in place over the cut surface of the distal femur. Hang the 4-in-1 cutting guide on a centrally placed pin on Whiteside's line located just below the cut slot for the anterior femoral resection. This cut slot location references the distal/anterior femoral cortex for proper anterior resection. The centrally placed pin may optionally be replaced by a protruding post located on the bone-contacting side of the 4-in-1 cut guide that fits into a corresponding hole in the femur.
Utilizing the proper sized 4-in-1 cut guide, this guide is now “rotated” until the posterior cut slot is parallel with the cut slot on the tibial cut guide.
Appropriate fixation pins/screws secure the femoral and tibial cut guides. All cuts can now be made, assuring proper patellar tracking.
Equal and rectangular gaps can be expected in both flexion and extension. Soft tissue releases are now performed to further balance compression forces in the medial and lateral compartments.
At least the following aspects of this disclosure are believed to be novel and non-obvious contributions over the prior art of knee arthroplasty:
Reference of distal anterior femur (DAF) and exact location of femoral head to accomplish exact knowledge of 1) varus/valgus of distal femoral cut, and 2) flexion/extension of anterior and posterior femoral cuts. Both data points are contained in the position of a distal femoral pin or hole.
Determination of proper External Rotation of femur by “hanging” the upper-center portion of a 4-in-1 femoral cutting block on the distal femoral pin, which is in the center of the trochlear groove. The proximal/distal axis through the center of the block is aligned with the longitudinal axis of the tibia, which aligns the trochlear groove of the femur (Whiteside's line) with the axis of the tibia at 90 degrees flexion of the knee. The distal femoral pin may optionally be replaced by a protruding post located on the bone-contacting side of the 4-in-1 cut guide that fits into a corresponding hole in the femur.
With proper ER of the femoral component, the posterior femoral cut and the proximal tibial cut will be parallel at 90 degrees knee flexion. Therefore the 4-in-1 femoral cutting block can be extended to a 5-in-1 cutting block by adding the proximal tibial cut slot.
The 5-in-1 (effective) block is attached superiorly (proximally) at the distal femoral pin or hole and distally to the tibial alignment rod extending to the middle of the ankle. The patellar will now track properly.
With the rectangular gap at the femur and tibia, equal compression will exist between medial and lateral compartments of the knee both in flexion and extension.
Other:
Finding the femoral head.
Bar fixed to operating table over the area of the femoral head with goal post marker/target.
Ultrasound method of locating femoral head.
Guide to reference DAF and then connect to femoral head goal post/target to determine distal femoral pin location. Arthroscopic procedure contemplated.
Adjustable 4-in-1 femoral cut guide.
This disclosure teaches bony and soft tissue preparation of the knee joint utilizing instruments and techniques consistent with proven total knee arthroplasty instruments principles.